This invention relates generally to the construction of concrete walls, slabs or other structures adjacent to or inclusive of spaces. More particularly, this invention pertains to void forms for creating spaces beneath concrete structures to separate and protect the structures from underlying expansive soils.
Expansive soils are prevalent in many areas of the United States, as well as in other countries. Such soils typically contain much clay, and expand and contract considerably as a result of cyclical changes in moisture content and/or as a result of natural freezing-thawing cycles.
A common method of construction in such expansive soils uses spaced drilled piers or spread footings for supporting the walls and floors. In this method, the concrete walls or beams supported by the piers or footings must be provided with a substantial spacing from the expansive underlying soil. Otherwise, the upward expansion of the soil may contract and force the beams or walls upward, causing cracking and deformation of the concrete. Without the required spacing, the integrity of the concrete structure is eventually lost.
Excavation of soil from beneath a concrete structure after it has "set" is a labor-intensive, very expensive method for resolving the problem with expansive soils. Where the structure has a lower edge below grade, a trench sufficiently wide to permit hand removal of soil below the structure must be provided. Furthermore, it is desirable to remove any forms of wood, metal or plastic used to form the lower surface of the structure. Such forms have a long life and should be removed after the concrete has set to provide further expansion space below the structure, and for re-use.
The use of integral corrugated paper form voids is known. In one application, such form voids are placed at the bottom of wall forms and trenches to separate the subsequently poured concrete from the ground. The corrugated paper form voids have sufficient temporary strength to support the wet concrete at a distance above the ground, but gradually absorb water and deteriorate to a condition where they no longer provide support. However, by this time the concrete has set and needs no support other than that provided by the piers. During periods of upward expansion of the underlying soil, the soil occupies the space left by the deteriorated or weakened form voids.
Form voids are available in various cross-sectional configurations. The generally rigid form voids are prepared by forming the desired structural shape of panels of corrugated paper and joining the panels together with adhesive. An internal cellular grid structure may be used within the form void to increase the strength of the form void as required.
It has been common practice to abut square-ended form voids to round piers, resulting in the flow of wet concrete downward between the pier and form void. While the quantity of this misdirected concrete is relatively small, it has been found that it nevertheless may provide a lower surface against which expanding soils may apply upward forces to distort or damage the concrete structure.